1. Field of the Invention
The present invention relates to a catalyst for purifying exhaust gases and a process for purifying exhaust gases. More particularly, it relates to the catalyst and the process which can efficiently purify nitrogen oxides (NO.sub.x) in the exhaust gases whose oxygen concentrations are at the stoichiometric point or more than required for oxidizing carbon monoxide (CO) and hydrocarbons (HC) therein.
2. Description of Related Art
As catalysts for purifying automotive exhaust gases, there have been employed 3-way catalysts so far which oxidize CO and HC and reduce NO.sub.x simultaneously to purify the exhaust gases. For example, the 3-way catalysts have been known widely which comprise a heat resistant support formed of cordierite, a catalyst carrier layer formed of gamma-alumina and disposed on the support, and a noble metal selected from the group consisting of Pt, Pd and Rh and loaded on the catalyst carrier layer.
The purifying performance of the 3-way catalysts for purifying exhaust gases depends on the air-fuel ratio (A/F) of automotive engine. For instance, when the air-fuel weight ratio is larger than 14.6, i.e., when the fuel concentration is low (or on the fuel-lean side), the oxygen concentration is high in exhaust gases. Accordingly, the oxidation reactions purifying CO and HC are active, but the reduction reactions purifying NO.sub.x are inactive. On the other hand, when the air-fuel ratio is smaller than 14.6, i.e., when the fuel concentration is higher (or on the fuel-rich side), the oxygen concentration is low in exhaust gases. Accordingly, the oxidation reactions are inactive, but the reduction reactions are active.
Moreover, when driving automobiles, especially when driving automobiles in urban areas, the automobiles are accelerated and decelerated frequently. Consequently, the air-fuel ratio varies frequently in the range of from the values adjacent to the stoichiometric point (or the theoretical air-fuel ratio: 14.6) to the fuel-rich side. In order to satisfy the low fuel consumption requirement during the driving conditions such as in the above-described urban areas, it is necessary to operate the automobiles on the fuel-lean side where the air-fuel mixture containing oxygen as excessive as possible is supplied to the engines. Hence, it has been desired to develop a catalyst which is capable of adequately purifying NO.sub.x even on the fuel-lean side (i.e., in the oxygen-rich atmospheres).
In view of the aforementioned circumstances, the applicants of the present invention applied for a Japanese Patent for a novel catalyst under Japanese Patent Application No. 4-130,904 (Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652). This catalyst comprises an alumina support, and an alkaline-earth metal and Pt loaded on the alumina support. With the catalyst, NO.sub.x is adsorbed on the alkaline-earth elements during the fuel-lean side (i.e., in the oxygen-rich atmospheres) driving. But, on the next fluctuation of the air-fuel ratio to the stoichiometric point or the fuel-rich side (i.e., in the oxygen-lean atmospheres), NO.sub.x is purified by reacting it with the reducing gases such as HC and the like. Thus, the catalyst is superb in the purifying performance of NO.sub.x even on the fuel-lean side (i.e., in the oxygen-rich atmospheres ). The catalyst proposed in Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652 is believed to provide the advantageous effect as follows: The alkaline-earth oxides, for example, Ba oxide loaded on the support, react with NO.sub.x to produce nitrates. Thus, NO.sub.x is adsorbed on the support of the catalyst in the form of the barium nitrate.
However, the exhaust gases usually contain SO.sub.2 which is produced by burning sulfur (S) contained in the fuel. Further, the catalyst ingredient oxidizes SO.sub.2 to SO.sub.3 in the oxygen-rich atmospheres (i.e., on the fuel-lean side). Still further, SO.sub.3 reacts readily with water also contained in the exhaust gases to produce sulfuric acid. When the resulting sulfuric acid is adsorbed on the support made from alumina, the alumina of the support becomes acidic, and the acidic alumina and the acidic NO.sub.x repel each other. Thus, NO.sub.x is inhibited from adsorbing an the barium. As a result, the catalyst proposed in Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652 suffers from a drawback in that it is deteriorated in terms of the NO.sub.x purifying performance after it is subjected to a durability test.
In order to improve the catalyst proposed in Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652 in terms of the NO.sub.x purifying performance, one can think of decomposing the produced sulfates so as to maintain the NO.sub.x adsorbability of the alkaline-earth metal. In order to decompose the sulfates, for example, aluminum sulfate, it is necessary to heat the catalyst to a high temperature of from about 600.degree. to 700.degree. C. and to hold it in a reducing atmosphere. Such a condition, however, is not practical for automobiles.
In addition, the alkaline-earth metal such as barium and the like is likely to react with alumina at high temperatures. Because of this undesirable property, the catalyst proposed in Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652 suffers from a drawback in that it is poor in terms of the heat resistance.